Traditionally an optical component, such as an optical glass lens element, has been produced from optical materials, such as crown or flint glass. The final finishing procedure has been performed by grinding and polishing in order to obtain desired lens element surfaces.
Various parameters that must be taken into consideration when forming lens elements from molten and heat-plasticized optical materials are well known, with U.S. Pat. No. 1,558,790 to Chalmers, U.S. Pat. No. 1,888,963 to Peiler, and U.S. Pat. No. 2,993,302 to Soubier, being examples. Chalmers discloses the need for weight control and the comparatively fine limits that must be maintained for slow flowing, rapid chilling optical materials. Peiler discloses the need for regulating the optical material mass, time, size and working conditions. Soubier discusses the need to control temperature, pressure, orifice dimensions, etc., in order to obtain the proper amount of molten material when forming a mold charge.
In some conventional molding methods optical materials are inserted into molds before being pressed between an upper mold and a lower mold having a shell barrel for confining the optical material between the upper and lower molds and for guiding the molds during the molding procedure. In press molding, where at least the lower mold has a convex surface, one routine procedure is to insert the optical material into molds by dropping it from a nozzle. There are advantages to using this drop method that forms a spherically-shaped agglomerate. The two most notable are the volume accuracy requirement of the produced lens element and the overall production cost. However, because the optical materials are almost spherical, they tend to be off balance and slip and shift to one side of the mold space when placed on the convex shaped lower mold surface. The optical materials will usually not stay at the center of the mold but will roll off of the raised central convex area and lodge against the mold barrel. When the optical materials slip and shift inside the mold space before pressing, irregularities often result. Thus, the lens element that is formed is often eccentric. A part of the optical material sometimes flows into the space between the lower mold and the mold barrel, causing difficulty in retracting the mold and optical component. It is also common to have stress or pressure irregularities form within the resulting optical component. On occasion it is even possible to damage the mold.
Measuring and charging molds with hot melt optical materials has been common for many years. U.S. Pat. No. 1,484,085 to Rule discloses using various size outlets for control of the size and shape of a charge used for molds. U.S. Pat. No. 1,873,021 to Peiler discloses control of the form and size of the hot preform charge to approximate the interior of the mold walls as closely as possible. The defects caused by surface chill and the resulting stretching and rupture of the chilled surface during pressing are also disclosed. U.S. Pat. No. 5,945,045 to Parker attempts to inject a precise, known shot volume of molten material into a mold.
In another method, the optical materials inserted into the molds are cut from plate glass. These are usually disc shaped preforms that have fixed planes that are relatively stable in the molds and stay at or close to the center of the mold. However, as the optical materials are cut from plate glass, it is difficult to provide each one with the same volume, and even more difficult to provide them with the same volume that the final optical component will have. Because of this, it is necessary that the lens elements be treated after molding with extremely accurate cutting and/or grinding, and followed by polishing if precision lens elements are to be prepared. When using flat, disc-shaped optical materials, the slip and shift within the mold space does not occur but the optical component shape was either incomplete or not obtained. In order to adjust the curvature and volume of the optical material so as to be identical to the desired lens element volume, an accurate grinding process was required. This resulted in the production process being more costly.
Many optical manufacturers use optical material preforms to make a final optical component. U.S. Pat. No. 2,532,501 to Johnson places a cold preform between dies and heats and presses to form an optical component. U.S. Pat. No. 3,396,214 to Crandon places a blank or optical preform in a mold that is almost in the final shape desired, and then forms a final coating on the blank or uses ultrasonic means to modify the blank. U.S. Pat. No. 5,662,951 to Greshes uses a molten preform mass and prefers to have excess material for later manipulation control. Because of the concave lower surface giving a point or line contact with the mold surfaces, air trapping is prevented.
Recently lens elements with comparatively small diameters have been produced by press molding. This method places the optical material into the molding unit and forms the lens element by compressing the optical material between the surfaces of the mold.